I just read in Mary Holland's quite excellent nature blog, "Naturally Curious." that we apparently don't need or even want to clean out old bluebird nests: Here's what she writes:
Contrary to what those of us who clean out our bluebird boxes have been led to believe, Cornell Lab of Ornithology states that experiments show preferences for nesting boxes containing old nests. In a paired experimental design bluebirds chose boxes containing old nests in 38 of 41 cases in which boxes with old nests were paired with empty ones. Scientists conjecture that this may be because the old nests often contain wasp larvae, an easy source of food for the bluebirds.